The present invention relates to multi-modality scanners. More specifically, the present invention is concerned with a detector assembly for multi-modality scanners, in particular but not exclusively, an APD (Avalanche PhotoDiode)xe2x80x94based detector for multi-modality PET(Positron Emission Tomography)/SPECT(Single Photon Emission Computed Tomography)/CT(Computerized Tomography) scanners.
The lack of anatomical information and the lack of information about the photon attenuation in the body in emission tomography imaging like SPECT (Single Photon Emission Computed Tomography) or PET (Positron Emission Tomography) imaging are major factors limiting the ability to accurately quantify radionuclide uptake in small Regions Of Interest (ROI). Such lack of information about anatomical retails and photon attenuation limits the diagnostic utility of emission imaging.
A drawback of emission tomography is that the spatial resolution obtained is limited. Typical emission tomographs have a resolution of the order of 5 to 15 mm. Another important drawback is that the images produced are very noisy since the doses of radioisotopes that can be injected and the maximum counting rate are both limited. These two drawbacks render the delimitation of the regions of interest difficult.
Transmission imaging presents the advantage to have a sub-millimetric spatial resolution and thus allows to uncover anatomical details of the organ of interest. A drawback of transmission imaging is that it provides very little functional information.
The trivial solution to overcome the above mentioned drawbacks of each imaging method is to gather CT (transmission) and SPECT or PET (emission) images and to merge the anatomical and functional information. A way to achieve this is to collect the two sets of data using two different apparatuses, one gathering anatomical information and the other functional information, and to co-register one set with the other using sophisticated software.
A drawback of the latter method is that the patient must be moved from one scanner to the other between the two scans. Moreover, the two scans will most often be obtained several days apart, subject to scanner availability. As a consequence, it is usually difficult to superimpose the two sets of data since the measurements are made separately, at different times, using scanners having two different geometries and two different resolutions. Movements of the patient as well as changes of the anatomy over time can cause significant mismatch between the resulting images.
A combined PET/CT tomograph has been developed jointly by the PET Facility at the Pittsburgh Medical center and CTI PET Systems Inc. This tomograph was described at the following World Wide Web address: T. Beyer, D. W. Townsend, T. Brun, P. E. Kinahan, M. Charron, R. Roddy J. Jerin, J. Young, L. Byars, R. Nutt, xe2x80x9cA combined PET/CT scanner for clinical oncologyxe2x80x9d, Journal of Nuclear Medicine 41(8):1369-79, 2000 Aug. , on Mar. 31st, 1998. The structure of the proposed tomograph allows acquiring sequentially anatomical (CT) and functional (PET) information and does not require the patient to be moved between scans.
A drawback of the latter tomograph is that it can be difficult to superimpose the two sets of data since the measurements are done separately, with two different resolutions and two different geometries. Moreover, movements of the patient as well as movements of the non-rigid structures within the body such as in the thorax or the abdomen add a blur to the resulting images.
The article entitled xe2x80x9cThe Design and Performance of a Simultaneous Transmission and Emission Tomography Systemxe2x80x9d, published in IEEE Transactions on Nuclear Science, Vol. 45, No 3, June 1998, and authored by Guliberg et al. proposes a simultaneous transmission and emission tomography system. The system includes a detector to collect data from transmission and emission sources at different energies, and two other detectors To simultaneously acquire emission data.
A drawback of the system of Gullberg et al. is that the incorporation of a transmission-computed tomography system into a three-detector SPECT system causes problems of transmission data truncation and crosstalk between transmission and emission data windows. Another drawback relates to the used detectors which are limited in count rate and which cannot collect sufficient number of events in a short time for accurate anatomical definition of the body structures. Furthermore, movements of the patient can still cause blur.
In U.S. Pat. No. 5,376,795, issued on Dec. 27, 1994, Hasegawa et al. describe an emission-transmission imaging system that uses a single detector for the transmission and emission data. The detector operates in a count or pulse mode to allow discrimination between the emission and transmission photons at low or medium count rates. At high count rate (with an X-ray tube, for instance) the detector can operate in a current mode without energy resolution. One advantage of this system is that both the transmission and the emission images are obtained by the same detector and. thus, are intrinsically aligned. One drawback of the system of Hasegawa et al. is that it cannot be used to produce PET emission imaging.
An object of the present invention is therefore to provide a multi-modality scanner that possesses none of the above mentioned drawbacks of the prior art.
Another object of the invention is to provide a detector assembly for multi-modality PET/SPECT/CT scanners.
More specifically, in accordance with the present invention, there is provided a detector assembly for multi-modality scanners for detecting low and/or high energy radiation emitted from a source under investigation. This detector assembly comprises a low energy radiation detector substantially transparent to high energy radiation but responsive to low energy radiation from the source. This low energy radiation detector therefore produces, in response to the low energy radiation, first radiation characterizing signals. The detector assembly also comprises a high energy radiation detector located downstream the low energy radiation detector and responsive to high energy radiation from the source. This high energy radiation detector produces, in response to the high energy radiation, second radiation characterizing signals.
Since the detector assembly comprises a low energy radiation detector substantially transparent to high energy radiation and a high energy radiation detector located downstream the low energy radiation detector, this detector assembly is suitable for conducting PET, SPECT and CT tomography either independently or simultaneously.
According to a preferred embodiment of the invention, at least one of the low and high energy radiation detectors comprises a scintillator, and the detector assembly further comprises a photodetector optically coupled to the scintillator. This photodetector produces, in response to the radiation characterizing signals from the scintillator, corresponding electric signals. The photodetector is advantageously selected form the group consisting of an avalanche photodiode, a pin diode and a photomultiplier tube.
In accordance with other preferred embodiments of the subject invention:
the low energy radiation detector comprises a high-luminosity scintillator, this high-luminosity scintillator preferably being a CsI(TI) scintillator;
the high energy radiation detector includes at least one high-density scintillator, this high-density scintillator preferably comprising a LSO scintillator and a GSO scintillator;
the LSO scintillator is located downstream of and is optically coupled to the CsI(TI) scintillator, and the GSO scintillator is located downstream of and is optically coupled to the LSO scintillator;
the low energy radiation includes X-rays, the high energy radiation includes high energy gammas, the low energy radiation further includes low energy gammas, and the high energy radiation further includeand low energy gammas.
The present invention further relates to a multi-layer detector assembly for multi-modality scanners for detecting X-rays and/or high energy gammas emitted from a source under investigation. The detector assembly comprises a first X-ray detecting layer substantially transparent to high energy gammas and producing, in response to X-rays from the source, X-ray characterizing signals, and a second high energy gamma detecting layer located downstream the first layer and producing, in response to high energy gammas from the source, gamma characterizing signals.
Preferably, at least one of the first and second layers is a scintillator and the characterizing signals produced by the scintillator include scintillation, and the multi-layer detector assembly further comprises a photodetector layer which produces, in response to the scintillation, a corresponding electric signal.
According to another aspect of the present invention, there is provided a multi-modality scanner comprising a gantry having a longitudinal axis. a scanner operation controller, and at least one pair of diametrically opposite detector assemblies as described hereinabove. The two diametrically opposite detector assemblies mutually face each other and are connected to the scanner operation controller whereby the first and second radiation characterizing signals from the detector assemblies are supplied to the scanner operation controller. The multi-modality scanner further comprises a low energy gamma radiation source movably mounted to the gantry and a collimator mounted rotatable to the gantry about the longitudinal axis. Both the low energy gamma radiation source and the collimator are connected to and controlled by the scanner operation controller.
The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings.